This invention relates to compact lenses for digital camera applications; in particular, for very compact digital cameras such as could be incorporated into a cellular telephone, personal digital assistant, or other very small electronic device.
Digital cameras utilizing high-resolution electronic imaging sensors require high resolution optics. For the consumer market, it is important that the lenses can be produced in high volume inexpensively. For use in very compact digital cameras, and cameras that might be incorporated into devices such as palm-sized computers, cellular telephones and the like, the lens must be very compact. In particular, it must have a very short length from the lens front surface to the image plane.
In the prior art, high resolution lenses have generally been made up of several individual lens elements in order to balance the inherent optical aberrations. These lenses that require a large number of elements tend to be relatively large, heavy, and expensive to manufacture. (The cost of these lenses increases with the number of elements, also resulting in increased costs in assembling and mounting them in a lens cell.) Prior lenses are generally designed using all spherical surfaces or using at least some aspheric elements in which one or both surfaces are non-spherical. Where all elements have spherical surfaces, generally a high number of lens elements is required, making the lens long and expensive to produce.
Aspheric lenses have some optical advantages, but cannot be easily produced by traditional glass grinding and polishing techniques. Aspheric elements are typically produced by molding plastics or low melt temperature glasses. While molded plastic elements are inexpensive to produce, the level of precision of the lenses is not always sufficient for high-resolution cameras, especially if a plastic element is used primarily as a focusing element. In addition, the optical properties of most plastic materials change with changes in temperature and humidity. The index of refraction of the plastic lens materials changes with changes in temperature, such as going in and out of doors on very hot or very cold days. This change is a significant problem with the focusing element(s), but is of much less consequence with other elements which primarily correct for aberrations. Lenses with all glass elements can overcome this problem, but tend to be large and excessively expensive for use in compact digital cameras used in other devices, such as an accessory built into a cellular phone.
Chemical film, as used in conventional film cameras, can be exposed with light coming from any direction, even at a low angle to the film surface. For digital cameras using inexpensive electronic imagers, to achieve optimum performance the light should contact the imaging media at angles of less than about 15xc2x0 to a line normal to the imaging media surface.
Prior lens designs generally have separate variable apertures and shutters, increasing the length of the lens assembly. Even where both these functions are combined in one device, that device must be positioned between lens elements because the aperture stops of conventional designs are located between lens elements.
Having the aperture stop between lens elements, as in the Double Gaussian designs, is believed to make correction of aberrations easier. Typical of such lens designs is that described by Fugii in U.S. Pat. No. 4,212,517, where the aperture stop is located between the third and fourth elements. This provides a degree of lens symmetry about the apertures stop, resulting in reduction in off-axis aberrations such as coma and distortion. It is generally believed that achieving good aberration correction without this symmetrical arrangement of lens elements would be difficult. However, it is difficult and expensive to integrate a variable aperture/shutter device with this type of optical design since it is difficult to keep the lens elements positioned precisely with the aperture device located between the elements.
Defuans, in U.S. Pat. No. 4,525,039, describes a lens design with the aperture stop in front of the first element. That design requires that the first element be plano-convex, with the plano surface facing the aperture. However, that design has a maximum relative aperture of f/4, too slow for use with cameras to be used at relatively low light levels. That design further requires seven elements, making it excessively long, heavy and expensive to produce for use in compact digital cameras.
Therefore, there is a continuing need for improved lenses that are not temperature sensitive, have excellent low-light performance and are compact, short, light weight and inexpensive to produce while using conventional, well-proven manufacturing methods.
The above-noted problems, and others, are overcome in accordance with this invention by a lens for digital cameras; in particular, such cameras that are incorporated into another device such as a cell phone, personal digital assistant and the like, that is extremely compact and has a short length from the front element surface to the imaging plane, have three lens elements with the front element formed from glass and the others from plastic and have excellent optical characteristics. For optimum results the lens has an aperture stop in front of the lens, external to the lens. If desired, an optional variable aperture/shutter can be positioned at the aperture stop position with precision.
The lens comprises three lens elements. The first, or front, lens element is a meniscus lens and is formed from a suitable glass by conventional lens grinding and polishing methods. Both surfaces of this lenses are spherical. The second and third elements are aspherical, formed from a suitable plastic by molding. Aspherical elements have at least one surface being a non-spherical surface. For optimum results, both surfaces of the aspheric elements are aspherical. An electronic imaging sensor is spaced a suitable distance from the rear element. Preferably, a cover glass is provided over the sensor surface. The use of a glass front element greatly reduces lens temperature sensitivity when the lens is taken from areas at great temperature differences, such as when bringing a camera into a building on a hot summer day or cold winter day.
Preferably, the aperture/shutter device is external to the optical elements so the optical elements can be assembled into a precision lens barrel independent of the aperture/shutter device. The performance of such a lens can be tested and verified before integration with an optional aperture/shutter device. Integration of such pre-assembled lenses with the aperture/shutter device can be performed with high reliability and repeatability, resulting in high yields in volume manufacturing.
It is, therefore, an object of this invention to provide a compact lens assembly particularly suitable for use in compact digital cameras, especially those incorporated into other compact electronic devices such as cellular phones, personal digital assistants and the like.
Another object of this invention is to provide a lens assembly for digital cameras that has very low sensitivity to changes in temperature.
A further object is to provide a digital camera lens having an extremely short length from the aperture/shutter device to the sensor imaging plane.
Yet another object is to provide a digital camera lens having a glass front element and two plastic elements to provide an optimum combination of imaging quality, small F-stop, and low manufacturing cost.